The term UD crossply laminate is used to indicate composite laminates of the known type with a plurality of layers of unidirectionally oriented parallel fibres (UD filaments) contained in a resin matrix, the UD filaments being arranged in different layers of crossing orientational directions, said layers being symmetrically positioned vis-a-vis a plane of symmetry through the centre of the laminate which runs parallel to its outer surfaces.
UD crossply laminates possess considerable advantages such as an improved surface quality, a comparatively low linear thermal coefficient of expansion (TCE) in the x and y directions, the option of incorporating a high content of fibres, and a favourable dimensional stability. In these respects UD crossply laminates are pre-eminently suitable as PWB substrate.
Such UD crossply PWB laminates are known from, int.al., U.S. Pat. No. 5,037,691 (Medney). The disclosed PWBs are manufactured by winding filaments about a square flat mandrel in several layers crossing at an angle of 90.degree., with the filaments being provided with curable matrix material by means of injection and/or impregnation. The matrix, which in consequence contains crosswise-applied layers of UD filaments, is then cured.
Other manufacturing methods in accordance with the above opening paragraph have been described in U.S. Pat. No. 4,814,945 (Leibowitz), EP 478 051, and WO 92/22191.
Leibowitz's disclosure relates to a PWB laminate comprising a matrix resin reinforced with parallel aramid fibres. The laminate is built up from layers of unidirectional aramid tape applied one on top of the other in crosswise fashion. The aramid tape is made by arranging a single layer of parallel aramid fibres to form fibre strips, coating the fibre strips with resin, and heating them to a semi-cured or "B"-stage.
EP 478 051 discloses the continuous manufacture of a flat substrate from a fibre-reinforced matrix, which process comprises the use of at least two moving layers of parallel, rectilinearly extending reinforcing fibres not bonded in the form of a woven fabric (UD fibres), providing said UD fibres, which are positioned in at least two crossing directions, with matrix material, and passing them through a laminating zone, for instance a double belt press, to form a crossply laminate.
In WO 92/22191 method of manufacturing a PWB laminate is described involving the steps of making non-flowable UD layers, coating at least part of the non-flowable UD layers with an adhesive on one or both sides, crosswise stacking the UD-layers in such a way that there is at least one layer of adhesive between each pair of UD-layers having a different direction of orientation, and bonding the stacked UD-laminates by activating the adhesive layers.
A general problem in the manufacture of UD crossply PWB laminates has to do with the step of providing the UD filaments with matrix resin. In order to fully benefit from the advantages of UD crossply laminates, the UD filaments should be orderly distributed over each UD layer. Problems may arise if portions of the filaments are not coated with matrix resin or if large resin areas are devoid of reinforcement. These problems include the occurrence of sink marks (highly undesirable surface unflatnesses due to differences in thermal expansion of portions of the laminate having different fibre volume fractions) and, in the case of PWBs made by additive plating, the occurrence of short circuits due to voids having become plated. This calls for an improved impregnation technique.
Another problem that particularly applies to the manufacture of UD-crossply laminates is that of disorientation of the UD filaments. In order to obtain a laminate having sufficient flatness, which is a property of particular importance to a PWB laminate, proper orientation must be retained. UD orientation is in jeopardy particularly when the resin is flowable under the conditions of lamination. For, on account of the flow which occurs during lamination, the tension, and hence the orientation of the UD layers, cannot be adequately controlled.
For economical and other reasons it is generally desired that UD crossply laminates can be made in a fast process. At the same time, such a process should be slow enough to allow proper impregnation to occur, and the resin should be sufficiently cured so as to have lamination under non-flow conditions. To this end it is known to employ fast curing resins that display good flow characteristics prior to curing. These usually are low-viscous, highly reactive resins. While the use of these resins may give a faster process than conventional resins, the high reactivity, and the consequently short potlife, can easily lead to curing at undesired stages in the process. E.g., curing will occur in the applicator unit in which the resin is held prior to its application onto the processing belt, or there is the danger of curing having proceeded too far before lamination, which is a disadvantage in that the strongest adhesive bonds between two UD layers are formed during lamination when at least one of the layers has not been fully consolidated.
As background art in the area of resins, not dealing with the manufacture of printed wire board laminates on the basis of UD filaments, int.al. EP 530 450 can be mentioned. This disclosure pertains to the continuous production of resin-impregnated materials by means of a double belt press. The resin is applied either via an extruder or, in the case of solid flakes, via a sprinkler. Further background art is JP 63/117053, which pertains to a thermosetting resin composition for printed circuit board laminates, which provides non-tacky prepregs by virtue of its melting point being above room temperature.